After installing XMail on my IPv6 server I had a problem where mail was received correctly and everything seemed to work correctly but messages got caught up in the spool folders. After investigation I found that the function resolving a hostname/IP Address from ASCII was failing. It turned out all I needed to do was add -M7 (see the readme) and that allows the function to work correctly so, if you use -S6 then make sure you also put -M7, -M6 or -M5 on your XMail command line.

Hope this helps someone as we move into the bold new world of 128bit IP addresses.